Hydroforming is finding increasing use as a method for creating complex shapes from simple tubes, without separate cutting and welding steps. For example, a passenger car subframe may be made from a single tube, rather than multiple pieces. A simple tube blank of consistent cross section is placed between a pair of dies that close over the tube to create a sealed cavity. The cross section of the cavity matches the final part shape desired. The interior of the tube is sealed and highly pressurized with a fluid, such as water, so that its outer surface is forced to take on the shape of the cavity.
It is often necessary that the part have various holes and openings, for fasteners, location features, etc. It is possible to punch or drill these holes subsequent to the hydroforming operation, but it would be obviously desirable to do it simultaneously, in-die. The current state of the art is defined by U.S. Pat. No. 4,989,482 to Mason. As disclosed there, a punch 6 is pushed through a close fitting bore in one of the dies, toward the outer surface of the formed tube, while the tube is still highly internally pressurized. The punch 6 has a cupped, sharp edged end face 7 that pushes through the surface of the tube, shearing out a slug 13 and pushing it inside the tube, leaving a round hole 15 in the tube. The punch end face 7 is ported to atmosphere, and it is claimed that this creates a suction cup action that will keep the slug 13 adhered to it. Once the slug 13 is punched out, pressurized fluid from the tube interior is exposed to the sliding clearance between punch 6 and its bore, which can cause a potential leak out of the die cavity and pressure loss. Pressurized fluid is also exposed through hole 15 to the interface between the tube outer surface 1 and the contacting inner surface of die 3, which can potentially prevent the tube from expanding fully out into its desired final shape. Only the metal to metal contact of the end of the punch 6 sticking through and engaging the rough edged hole 15 would prevent these two potential leaks, and that is not a compliant or secure seal. Even that poor seal would be broken once the punch 6 was withdrawn. In fact, once the tube interior is pierced, a third potential leak path is created, past the slug 13 and through the ported punch face 7. Again, only the metal to metal seal of the adhered slug 13 to and against the punch face 7 would prevent such a leak, which is an unreliable seal at best. The patent does not explicitly mention such potential leaks. Nor does it deal with ejection of the slug 13, which would fall into the interior of the tube and would require a separate removal step once the formed tube was removed. This can be a very difficult operation with a complexly formed tube, and slugs can easily stick inside the wet tube.
The main concern of the Mason patent is not sealing or slug removal, but another inevitable problem with inwardly directed punching: so called countersinking. As the punch 6 is pushed through the tube wall, an annular area 14 of the tube surrounding the punched hole 15 is countersunk inwardly. Indeed, the slug 13 cannot be punched out otherwise. The patent claims that, since the slug 13 is adhered to the ported punch face 7, retracting the punch 6 will pull the adhered slug 13 back against or partially into the rough edge of the hole 15 that it left behind, sealing hole 15 enough that pressurized fluid can then push back and at least partially flatten the countersunk area 14. Again, reliance on a metal to metal seal, especially of rough, deformed edges against one another, is questionable. Even in the best case, the countersunk area is not completely flattened back out, nor is the edge of hole 15 sharp, nor is it claimed to be. Consequently, most pierced in the die holes in hydroformed parts will be found to have a very noticeable countersunk area, and the slug will be found to be hinged to one side of the hole, not completely sheared away.
The other possible method of in-die hole piercing is to allow the pressurized fluid to shear its own slug by blowing it outwardly, into a sharp edged cutting edge in the die. This leaves a sharp edged hole without countersinking, but presents its own problems in terms of timing, sealing, and slug ejection. If a hole is pierced too soon, pressurized fluid can leak into the tube die interface and prevent the tube from forming completely out into the die cavity, as noted. Once a hole has been pierced, it must be effectively sealed against the escape of pressurized fluid from inside the tube. And, the slug will be forced into the interior of the cutter, unless it is ejected back into the tube, and must be removed somehow as a later step. These problems unique to in-die piercing have not been adequately resolved to give a truly practical, production feasible system.
One fairly old reference, U.S. Pat. No. 3,495,486 to Fuchs, does disclose an apparatus that uses pressurized fluid to punch holes in a tubular member by blowing a portion of the tube through a sharp edged cutter. However, the tubular member disclosed is one that is already formed to shape, with a rectangular cross section, so the apparatus is really concerned only with hole forming per se, not tube forming. The problems unique to hydroforming at the same time as hole piercing, described above, are not faced or dealt with, and the apparatus shown could not be used practically in a hydroforming method. Fuchs shows two apparatuses, one in which the slug is blown outwardly, and one in which it is actually blown inwardly into a cutter, FIG. 9. In each case, the sharp edged cutter is provided in a structure separate from the main dies. In FIG. 8, where the slugs are blown outwardly, the cutters (118) are provided in a sleeve 114 that slides inside a cavity in a die block 111 and over the entire outer surface of the tube 110. In FIG. 9, where the slugs are blown inwardly, the cutters are provided in a mandrel 184 that fills the entire interior volume of the tube 110. The FIG. 9 apparatus, therefore, would be totally impossible to use in a hydroforming method, where the tube interior must be empty.
In the FIG. 8 apparatus, Fuchs does deal with the problem of controlling the timing of the slug blowout by providing a slidable back up plunger 138 within the cutter, which holds the tube surface back until sufficient piercing pressure is reached, then retracts it to allow slug blow out to occur. However, it must be kept in mind again that the pressurized fluid is needed only to blow out the slug, and not to hydroform the tube into its final shape as well. Therefore, the same sealing problems are not faced. There is no provision to keep pressurized fluid from leaking into the interface between the tube and surrounding sleeve. The only seal that is even an issue is the one needed, once the slug 156 is broken out, to keep pressurized fluid from escaping out of the tube interior and behind the plunger 138. Here again, as in the Mason patent, there is a hopeful reliance on the fact that the slug 156 will be tightly wedged into the cutter 118 to prevent leakage. While the slug 156 would indeed be tightly wedged and bowed inside the cutter 118, the metal to metal contact of the rough edged slug 156 against the inside of the cutter 118 would not provide a reliable seal, especially against very high pressure.
Another impediment to applying the Fuchs apparatus to in-die hole piercing is the problem of slug removal. Once the slug 156 is wedged tightly into the cutter 118, it will be difficult, if not impossible, to eject it by reversing the back up plunger 138, which is the ejection method described. Even if the slug 156 is successfully ejected, it will be pushed back inside the tube 110, or back into the sleeve 114, as the Fuchs patent claims. The sleeve 114 cannot be pulled without dismantling the whole apparatus, however, and having to remove a slug from inside a formed tube is not practical, as already noted. In short, nothing about the Fuchs apparatus, but for the broad idea of controlling the timing of the slug blow out with a back up plunger, can be practically applied to in-die hole piercing in the hydroforming context. A new apparatus dealing with those unique problems would be needed.